Electrolytic device



Dec. 19, 1939. P. ROBINSQN ET AL 2,183,776

' ELECTROLYTIC DEVICE Filed May 27, 1938 LEAKAGE CURRE N Q a 150 250 350400 VOL rs PRESTON ROBINSON- & JOSEPH L. COLL/NS //v VENTORS ammy, Cod.1

A TTORNEYS Patented Dec. 19, 1939 UNITED STATES PATENT OFFICE Collins,

North Adams, Mass., Sprague Specialties Co.,

assignors to North Adams, Mass a. corporation of MassachusettsApplication May 27, 1938, Serial No.'210,533

9 Claims.

The present invention relates to electrolytic devices and moreparticularly to electrolytic condensers having novel and quite uniquecharacteristics.

This application is a continuation in part of applicants copendingapplication Ser. No. 743,469, filed September 10, 1934, now Patent No.2,122,393 and reissued as Re. 21,088.

It is a known phenomenon that when the volt- 10 age applied to anelectrolytic condenser exceeds a certain limit, which as a rulecorresponds approximately to the maximum voltage to which the condenseris formed, a spark discharge occurs at the film of the filmed electrode(or filmed electrodes) of the condenser, and at the same time theleakage current, which up to this sparking voltage is of a low value,sharply increases.

This spar 'ng voltage of the condenser has been regarded as the limitingvoltage, above which the condenser could not be operated. Not only isthe sparking accompanied by an objectionable noise, but the sparking maygreatly damage the film, both due to the mechanico-electrical effectcharacterizing sparking and because of the great as heat developmenttaking place directly in the vicinity of the film. This mechanism ofsparking, as is well known, is the successive building up of a highvoltage and the subsequentdischarge across a spark-gap, which in thepresent instance is formed between the electrolyte and the film or themetallic surface disposed beneath the film. The mechanism of sparkingand the relationship it bears to the present invention will be more asfully described later.

For the above reasons it has been the practice to operate electrolyticcondensers below the sparking voltage.

While to some extent, because of their well- 0 known self-healingproperties, electrolytic condensers can withstand, without detrimentaleifects transient voltages, which exceed their sparking voltage, such isonly the case as long as the occurrence of such over-voltages is infre-5 quent and of very short duration. In such cases no substantialcumulative heat development takes place and no substantial damage to thefilm is caused, and the small deteriorations of the film so caused, canbe repaired by the selfhealing action of the condenser.

However, in any application in which an electrolytic condenser is calledupon to stand a voltage exceeding its normal operating voltage forperiods of a few minutes or even a few seconds (or rapidly succeedingover-voltages of even shorter duration), the condenser has to bedesigned for this higher voltage.

For instance, as will be more fully discussed later on, in radio sets,certain electrolytic condensers operate normally at about 250 to 300 5volts; however, for a period of a few seconds after the set has beenturned on, these condensers have to withstand voltages of 450 to 500volts.

In the past such condensers had to be designed for these higher,short-duration voltages, which 10 meant that they had to be formed atthe higher voltages.

However, the formation of a 500-volt condenser is considerably costlierthan that of a 300 volt condenser. Furthermore, as is well 15 known, thecapacity per unit of surface area decreases roughly, in proportion tothe increase in forming voltage, and thus for a given capacity theelectrode surface required is about or about 1.66 times as great whenformed at 500 0 .volts, as when formed at 300 volts. The over-alldimensions of the condenser, and to a great extent its cost, increase ina similar manner for the higher voltage condensers.

The condensers according to our invention have the unique characteristicof altogether lacking a readily observable sparking voltage. Similarlyto standard condensers, they do exhibit a sharp increase of the leakagecurrent when the operating voltage exceeds a given critical voltage, butthis increase of current is not accompanied by sparking, nor by asubstantial local heating up of the film surface. Or in other words,there is no breaking-down of the film at a large number of individualpoints asis characteristic of spark discharges, but merely the blockingaction or insulating resistance of the film drops down uniformly to avalue considerably lower than that which it possesses below thiscritical voltage.

Consequently, the film can stand, without be- 40 mg damaged, a voltageexceeding its critical voltage for any reasonable length of time, andeven if a higher voltage is applied for several hours to the condenserand the condenser be ultimately damaged, this is because of the highcurrent passing the condenser unduly heating up the condenser as awhole. If the condenser is to normally stand such over-voltage for quiteextended periods, this can be taken care of by a more ample design ofthe condenser as a whole,

whereby, however, only a comparatively small part of the aboveadvantages obtained by the lower forming voltage, needs to besacrificed.

The condenser of our invention, has a capacity corresponding to that ofa condenser formed at or near the critical voltage referred to above,although it need not necessarily have actually been formed at thisvoltage. For example, the film may be formed initially to a highervoltage than the critical voltage, and then be attacked, for example, bytreatment in an alkaline solution as described'in our Patent No.2,035,022. Such a condenser when operated at or near the criticalvoltage will then assume the same capacity as if it had been formed atthe critical voltage initially.

Thus while the critical voltage in both the condensers, of the prior artand those made in accordance with our invention represents 21. voltagevalue above which the leakage current of the condenser sharply increasesand at which its voltage current curve thus shows a pronounced bend orknee, in condensers of the prior art at or above such a critical voltagesparking takes place, whereas in our condensers no sparking takes placeat the critical voltage, and furthermore, as a rule, the leakage currentincrease at this voltage is much sharper than it is in prior artcondensers.

Another distinction between our condensers and those of the prior art isas follows: When an electrode is operated in an electrolyte capable offorming it to a higher voltage (as in prior art condensers) and avoltage in excess of the original forming voltage is applied to thecondenser, the electrode immediately begins to form to the highervoltage, and the initially large leakage current decreases as does thecapacity of the condenser. In the case of our novel condenser, theleakage current remains substantially unchanged at its initial largevalue, or may even increase with time, without the condenser forming tothe higher voltage or reducing its capacity.

Thus a condenser in accordance with our invention besides the absence ofsparking, is also characterized by showing above the bend in itsvoltage-current characteristic curve, a high stable leakage current anda stable capacity.

We believe that the unique characteristics of the condensers of ourinvention may be explained as being brought about substantially in thefollowing manner:

As is well known, the effective dielectric in electrolytic condensersconsists of a thin dense film formed on the surface of the electrode,which dielectric film has a thickness corresponding to the criticalvoltage to which it is formed and to obtain the best results should beof high density and consist of substantially pure unhydrated oxide.

Such dielectric film contains .minute pores or intercrystalline passageswhich, when the condenser is subjected to a voltage in excess of itscritical voltage become filled with gas and behave to form minute sparkgaps between the electrolyte and the electrode. When these gaps arebroken down they form the centers of localized heat which tends todestroy the dielectric properties of the oxide film in their immediatevicinity. Moreover, when such gaps break down they produce thecharacteristic noise which in the art has come to be known asscintillation.

In the condenser of our invention the formation of these minute sparkgaps is prevented and no localized breakdown of the dielectric-film cantake place. One method of preventing the formation, of such spark gapswhich we have found is to provide on the dielectric film a thin layer ofhydrated aluminum oxide or aluminum hydroxide which effectively blocksthe surface of the film and fills any minute pores contained therein.Thus with such a blocked dielectric film no gas can accumulate thereinand localized breakdown cannot occur.

This hydrated layer may be formed electrolytically in a suitableelectrolyte or may be formed by chemical transformation, ire. hydrationof the outer surfaces of the dense unhydrated oxide layer previouslyformed on the aluminum surace.

Since the density of the unhydrated aluminum oxide film is of the orderof magnitude of 3.5, and the density of aluminum hydroxide isapproximately 2.4, whereas the molecular volumes of these substances arein the ratio of approximately 1 to 2 respectively, it is evident thatthe conversion of the aluminum oxide to aluminum hydroxide produces anincrease in volume of such magnitude that the conversion of only a smallamount of unhydrated aluminum oxide is required to substantiallycompletely fill up the pores contained in the film. Because the amountof unhydrated oxide which is converted is so small, the effectivethickness of the dielectric layer is not materially aifected.

A condenser in accordance with our invention may also be obtained byoperating a filmed electrode provided with a dense unhydrated oxide dielectric film, in an electrolyte which is incapable of forming gooddielectric film.

The characteristics of this operating electrolyte are such that itimparts to the film or to any gas bubbles which may form in the pores ofthe film, a negative zeta potential having a low value as compared tothe potential imparted both to the film and to such gas in the ordinarygood" forming electrolytes.

In its action a so-called good forming electrolyte imparts a strongnegative potential to the film, which means that as the oxide is formedit tends to strongly adhere to the anode surface. At the same time anygas formed will likewise have a strong negative potential and tends tostay in the pores of the film. Thus, when the voltage applied to acondenser provided with such a good electrolyte exceeds its criticalvoltage, breakdown through the gas bubble takes place and produces thedeleterious results previously described.

With an electrode operated in accordance with our invention, the filmhas only a slight negative zeta potential. Furthermore, any gas formedduring operation, having also a low negative potential, isliberatedeasily and freely and escapes from the film surface withoutsuch gas accumulating in the pores to form such previously describedspark gaps and no sparking can take place.

Since the formation of a stable dielectric in an electrolyte whichimparts to it a low zeta potential is extremely difllcult no appreciableunhydrated film-formation will take place as contrasted to the formationin a good" forming electrolyte as previously described. For this rea--son it is generally desirable to initially form the dielectric film onthe electrode in one of the socalled good electrolytes.

The distinction between condensers of our invention and those of theprior art may be substantiated by observing the behavior of suchcondensers under pressure. In an ordinary condenser, increasing theexternal pressure on the electrolyte, increases the pressure on the gasin the pores of the film, and it can be readily observed that thesparking voltage of the condenser increases. Conversely, lowering thepressure lowers the sparking voltage of the condenser. However, in bothinstances, sparking is present and may be readily seen.

In the condenser of our invention, there is no sparking, and increasingthe pressure merely increases the leakage current. This is explained bythe fact that under pressure, the electrolyte displaces the gas formedand permeates into the pores of the film thereby lowering its effectiveresistance.

The type of electrolyte found to impart the unique characteristics tothe condenser of our invention is, as above stated, one which imparts alow negative potential to the film and to the gas formed at the surfaceof the electrode. We have found that the desired lowering of thepotential may be obtained by increasing the concentration of theelectrolyte, i. e. increasing the number of positive and negative ionsavailable in the solution.

The potential produced by the electrolyte may also be controlled byvarying its pH value and/or by proper selection of the ions made toexist in the solution.

A process suitable for the formation of electrolytic condensers havingthe aforesaid unique characteristic is described in detail in ourcopending application Ser. No. 743,468, filed September 10, 1934 nowPatent No. 2,122,392. It should, however, be understood that the processof said application is not limited to the manufacture of the specialcondensers forming the subject matter of this application. On the otherhand, these special condensers can be manufactured also by otherprocesses, the general characteristics of which are that they produce afilm which comprises adjacent to the aluminum a layer which is ofsubstantially pure unhydrated oxide, and has a thickness correspondingto the critical voltage, and preferably also comprises an adjacent layerformed of a mixture af' aluminum oxide and aluminum hydroxide or othercompounds of aluminum. These processes, however, do not form a part ofthe present application.

In the process described in the application Ser. No. 743,468, theelectrode or electrodes of the condensers are subjected to a two-stepforming process, each forming step being a rapid formation step, as morefully described in the U. S. A. Patents No. 2,057,314 and No. 2,057,315to Preston Robinson.

According to the process described in our above application, in thefirst forming step the electrode is formed in an alkaline electrolyteand in the second step in an acidic electrolyte. In the first step theformation takes place by immersing into the electrolyte successiveunfilmed portions of the electrode and applying thereto immediately themaximum forming voltage. This voltage, for instance, for condensers inwhich the critical voltage is to be 300 volts, will be about 300 volts,although under certain circumstances the forming voltage may vary tosome extent from such critical voltage. In the second step, the formingvoltage is preferably the same as in the first step, but it is notaltogether necessary to gradually immerse the electrode as the electrodeis already filmed. As a rule the second forming step requires about 15to 30 minutes. This time, however, is not critical. 1

We shall describe our invention on hand of a specific example and inconnection with a socalled wet electrolytic condenser for radio filtercircuits, for which it is especially important.

However, it should be well understood that our invention is broadlyapplicable to various types of electrolytic condensers.

In the drawing forming part of the specification:

Figure 1 is a schematic diagram showing a filter circuit of a radioreceiving set utilizing condensers of our invention;

Fig. 2 is a graph illustrating the voltage-leakance with our invention,is described in detail in our copending application Ser. No. 743,468.

Both of the formation steps described in said application preferably,but not necessarily, take place before the assembly of the condensers,and usually a plurality of electrodes are formed simultaneously.

In the first forming step the electrode is immersed in an alkalineelectrolyte which preferably comprises as ionogen an alkaline salt of aweak acid, for instance, borax, sodium-phosphate, etc. The solution usedis preferably a very dilute aqueous solution of such ionogen. Forcondensers to be formed at 300 volts we may use, for instance, asolution comprising two ounces of borax to three gallons of water. I

The electrodes are gradually immersed in the electrolyte with theimmediate application of the full forming voltage, for example, 300volts. Thereby, as has been fully described in the U. S. A. Patents No.2,057,314 and No. 2,057,315 to Preston Robinson, the film forms almostinstantaneously on successive unfilmed portions of the electrode as theyimmerge into the electroytc.

This formation takes place at extremely high current densities, which,together with the high voltage, causes an exceedingly high electrostaticequivalent pressure at thefilm and this high pressure and rapidformation forms a very dense and unhydrated oxide film on the electrode.The temperature of the forming bath in this step should not exceed about50 C.

A specific characteristic of this forming step, as has been described indetail in our above said application, is that contrary to usual formingprocesses, there is no chemical reaction outside of the filmformation, 1. e., the usual production of reaction products in theelectrolyte, for instances, of aluminum oxide and boric acid, isentirely absent.

The film formed in this step on the aluminum electrodes has minutepores. One of the purposes of the second formation step is to convert aportion of the dense unhydrated film into a film which is less dense,more fibroid and elastic which covers and/or fills such pores.

The second forming step consists in immersing the filmed electrodes intoan acid electrolyte comprising, for instance, for 300 volt condensers, 1lb. borax, 4 lbs. boric acid, 6 gals. water, the forming electrolytepreferably having a temperature of 80 C. or more. Again,the-above-referred to rapid formation process is preferably used; otherweak acids as phosphoric, citric,

tartaric acid with or without the addition of salts of a weak acid mayalso be used.

In this second forming step the aluminum oxide film reacts at itssurfaces with the acidic constituent of the electrolyte.

The filming electrode so formed is then assembled into a condenser witha suitable electrolyte, usually an aqueous solution of a weak acidand/or the salt of a weak acid, whereby the salt of the weak acid doesnot need to be the salt of the acid used. Such weak acids are, forinstance, boric acid, phosphoric acid, citric acid, tartaric acid, etc.,and the salts used are generally alkaline metal or ammonium salts ofsuch weak acids.

The electrolytes using such constituents are usually to be of fairlyhigh concentration. For use with 300 volt condensers we have found thefollowing composition to give excellent results:

Boric acid 127 grams per liter of water 28% ammonia 22 cos. per literThis electrolyte has a low resistivity of the order of 50 ohms percentimeter cube at 25 C. and a pH of approximately 6.9.

Equally as suitable is an electrolyte composed of Boric acid 137 gramsper liter of water Sodium hydroxide 13 grams The resistivity of thiselectrolyte is of the order of 85 ohms per-centimeter cube at 25 C. andits pH is between 6.9 and 7.

The other electrode of the condenser may form the container and may beof aluminum or other filming or non-filming metal.

The advantages of our novel condenser will be described on hand of atypical example:

Figure 1 is a schematic circuit diagram of the power supply of a radioreceiving set. The regular A. C. lighting current is transformed to theproper voltage and then rectified and filtered to supply the platecurrent for the tubes of the set. The rectifier l is shown as afull-wave rectifier, the input side of which is connected to the windingof the transformer 20.

The leads l and H supply the rectifier and smoothened current to theplate circuits of the tubes.

The filter system provided between the rectifier and the output consistsof two choke coils and 6, connected in series in lead I0 and of threecondensers 2, 3, and 4. Of these condenser 2 is connected across leadsI0 and II directly behind the rectifier; condenser 3 is connected,

across these leads between choke coils 5 and 6; and condenser 4 isconnected across the leads l0 and I I in the rear of choke coil 6. Weshall consider primarily condensers 2 and 3.

The output or load of the rectifier which consists of the sum of theplate currents of the tubes of the radio set is in most of the sets ofthe order of 1000 milliamperes, whereas the normal output voltage is innormal operation usually 250 to 300 volts.

The inherent characteristics of the rectifiers most widely used aresuch, that their voltage output decreases with increasing load. Forinstance, in the normal vacuum type of rectifier tubes at zero load, thevoltage output of the tube is about 450 to 500 volts, whereas at a loadof 100 milliamperes it is about 250 to 300 volts.

As is well known, the plate current through the tubes of the set onlystarts to fiow when the cathode of the respective tubes has been broughtto their proper electron-emitting temperature.

In modern sets using indirectly-heated cathodes, the time required forthe cathodes to attain their full electron-emitting temperature isusually of the order of 10 to 25 seconds. Consequently at the instant aradio receiving set is put in operation, practically no current flowsthrough the rectifier and thus a high voltage is delivered by therectifier tube and the same high voltage exists across the condenser 2.

For this reason, as has been previously more fully explained, thecondensers used for this purpose, had in the past to be made for 450 to500 volts, in spite of the fact that in normal operation they operateonly at 250 to 300 volts.

On the other hand, condensers according to the invention, formed for 250to 300 volts, can be employed for this purpose without any damaging ofthe condensers. Thereby initially when the set is switched on, a currentof considerable magnitude fiows through the condenser. For example, a 16mid. condenser formed according to our invention for 300'volts operationwill draw a current of the order of 40-50 milliamperes at a voltage ofabout 400 volts, without localized breakdown of the dielectric filmtaking place.

After 10 to 25 seconds as the radio tubes assume their electron emittingtemperature and plate current starts to flow, the rectifier loadincreases and its voltage as well as the voltage across the condenser isreduced to about 250 to 300 volts. Under these normal operatingconditions the leakage current through the condenser decreases to itssmall normaloperating value, which is of the order of a fraction of amilliampere.

In the specific example both condensers 2 and 3 have been formed for 350volt'operation, condenser 2 having 8 mid. and condenser 3, 16 mid.capacity. When the set'is switched on a voltage of about 450 volts isapplied across condenser 2 and passes therethrough a current of about 23milliamperes, and a voltage of. 410 volts is applied across condenser 3and passes therethrough a current of about 40 milliamperes. Graduallythe voltage across these condensers drops below 350 volts and as thevoltage drops the leakage current drops down to a negligible value.

Fig. 2 shows the general shape of the curve of the leakage current asfunction of the voltage for a condenser formed at 300 volts according toour invention. As will be noted, at the critical voltage there is asharp bend or knee in the curve. This critical voltage, as has beenstated before, however, does not need to be the forming voltage.

It should be well understood that our invention is not limited to wetelectrolytic condensers, nor to condensers used in filter circuits, but

can also be applied to so-called dry" electrolytic condensers, as wellas to A. C. condensers.

Therefore, we do not wish to be limited to the application and exampledescribed, but desire the appended claims to be construed as broadly aspermissible in view of the prior art.

What we claim is: i I

1. An electrolytic condenser comprising a filmed electrode having aneifective dielectric film formed at a maximum forming voltage of severalhundred volts and adjacent to the surface of the electrode consisting ofsubstantially unhydrated aluminum oxide, and an electrolyte incapable offorming an unhydrated film on the electrode, said condenser whenoperated at voltages exceeding its critical voltage, not exhibitingsparking.

2. An electrolytic condenser comprising an aluminum electrode having aneffective dielectric film formed at a maximum forming voltage of severalhundred volts, said film adjacent to the surface of the electrodeconsisting of substantially unhydrated aluminum oxide, and anelectrolyte incapable of forming an unhydrated film on the electrode,said condenser when subjected to a voltage exceeding its criticalvoltage exhibiting a large increase in leakage current withoutaccompanying sparking phenomenon.

3. An electrolytic condenser comprising a filmed electrode having aneffective dielectric film formed ,at a maximum forming voltage ofseveral hundred volts, said film consisting adjacent to the aluminum ofsubstantially unhydrated aluminum oxide, and an electrolyte incapable offorming'an unhydrated film on the electrode, said condenser whenoperatedat voltages exceeding its maximum forming voltage not exhibitingsparking.

4. An electrolytic condenser comprising an aluminum electrode having aneffective dielectric film formed at a maximum-forming voltage of theorder of 250 to 350 volts, said film consisting adjacent to the aluminumof substantially unhydrated aluminum oxide, and an electrolytecomprising an aqueous solution of a weak acid and a salt of a weak acid,said electrolyte being incapable of forming an unhydrated filmon thealuminum, said condenser having at a voltage exceeding said maximumforming voltage, a much increased leakage current and exhibiting nosparking.

5. An electrolytic condenser comprising a filmed electrode formed at amaximum voltage of the order of several hundred volts, said filmadjacent to the surface of the electrode consisting of substantiallyunhydrated aluminum oxide, and an electrolyte incapable of forming anunhydrated film on the electrode, said condenser being characterized byits lacking a sparking voltage.

6. An electrolytic condenser comprising an electrolyte and an aluminumelectrode immersed in said electrolyte, said electrode provided withaluminum a layer of substantially unhydrated aluminum oxide adjacent tothe surface of the electrode and adjacent to said unhydrated aluminumoxide a layer of hydrated aluminum oxide effectively blocking thesurfaces of said unhydrated layer, said electrolyte being incapable offorming an unhydrated film on the electrode, said condenser beingadapted to operate at a greatly increased leakage current at a voltageexceeding that at which said electrode has been formed withoutexhibiting sparking.

7. In an electrolytic condenser, an aluminum electrode provided with asubstantially unhydrated aluminum oxide film and an electrolyte whichimparts a substantially zero negative zeta potential to said film.

8. An electrolytic condenser comprising an aluminum electrode having aneffective dielectric film formed at a maximum forming voltage of theorder of several hundred volts, said film consisting adjacent to thealuminum of substantially unhydrated aluminum oxide, and an electrolyteincapable of forming an unhydrated film on the electrode and comprisinga weak acid and a salt of a weak acid, said electrolyte having aresistivity of less than 250 ohms per centimeter cube at 25 C., saidcondenser having at a voltage exceeding said forming voltage a muchincreased leakage current and exhibiting no sparking.

9. An electrolytic condenser comprising an electrode having an efiectivedielectric film formed at a maximum forming voltage of the order ofseveral hundred volts, said film consisting adjacent to the aluminum ofsubstantially unhydrated aluminum oxide, and an electrolyte incapable offorming an unhydrated film on the electrode and comprising a weak acidand a salt of a weak acid, said electrolyte having a resistivity of lessthan 250 ohms per centimeter cube and a pH between 6.7 and 7.1 at 25 C.,said condenser having at a voltage exceeding said forming voltage a muchincreased leakage current and exhibiting no sparking.

PRESTON ROBINSON. JOSEPH L. COLLINS.

